mmap不存在物理地址错误

Question

我正在尝试mmap一个文件,它似乎成功但是当我访问它时,我收到一个SIGBUS错误.码:

int main(int argc, char* argv[]) {
int pid = getpid();
char cmd [1024];
int file;
if (argc != 2) {
    printf("Wrong arguments\n");
    return 1;
}
printf("%s", argv[1]);
int numpages = atoi(argv[1]);
sprintf(cmd, "cat /proc/%d/maps", pid);
system(cmd);
long page_size = sysconf(_SC_PAGESIZE);
file = open(argv[1], O_RDWR, (mode_t)0600);
ftruncate(file, page_size  * numpages);
if(file == -1) {
    perror("file open failed!\n");
    return 1;
}
printf("\n\n mapping file - numpages: %d \n\n", numpages);
printf("mapping %ld KB\n", page_size  * numpages/1024);
int* mapped_file = mmap(0, page_size  * numpages,  PROT_READ | PROT_WRITE, MAP_PRIVATE, file, 0 );
if (mapped_file == MAP_FAILED) {
    perror("Map failed");
    return 1;
}
printf("mapped file at %p\n\n", mapped_file);
int* current_pos = mapped_file;

system(cmd);

int j;
for (j=0; j<numpages; j++) {
    printf("%d   ", *current_pos);
    current_pos += page_size/4;
}
if (munmap (mapped_file, page_size  * numpages) == -1) {
        perror ("munmap");
        return 1;
}
if (close(file) == -1) {
    perror("close");
    return 1;
}
return 0;
}

输出:

(gdb) run 64
Starting program: /home/jords/engcode/workspace/SE370Assignment3/A3Program3 64
00400000-00401000 r-xp 00000000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
00600000-00601000 r--p 00000000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
00601000-00602000 rw-p 00001000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
7ffff7a48000-7ffff7bd2000 r-xp 00000000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7bd2000-7ffff7dd1000 ---p 0018a000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7dd1000-7ffff7dd5000 r--p 00189000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7dd5000-7ffff7dd6000 rw-p 0018d000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7dd6000-7ffff7ddc000 rw-p 00000000 00:00 0 
7ffff7ddc000-7ffff7dfd000 r-xp 00000000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7ffff7fd0000-7ffff7fd3000 rw-p 00000000 00:00 0 
7ffff7ff8000-7ffff7ffb000 rw-p 00000000 00:00 0 
7ffff7ffb000-7ffff7ffc000 r-xp 00000000 00:00 0                          [vdso]
7ffff7ffc000-7ffff7ffd000 r--p 00020000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7ffff7ffd000-7ffff7fff000 rw-p 00021000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7ffffffde000-7ffffffff000 rw-p 00000000 00:00 0                          [stack]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
64


mapping file - numpages: 64 

mapping 256 KB
mapped file at 0x7ffff7ff4000

00400000-00401000 r-xp 00000000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
00600000-00601000 r--p 00000000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
00601000-00602000 rw-p 00001000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
7ffff7a48000-7ffff7bd2000 r-xp 00000000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7bd2000-7ffff7dd1000 ---p 0018a000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7dd1000-7ffff7dd5000 r--p 00189000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7dd5000-7ffff7dd6000 rw-p 0018d000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7dd6000-7ffff7ddc000 rw-p 00000000 00:00 0 
7ffff7ddc000-7ffff7dfd000 r-xp 00000000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7ffff7fd0000-7ffff7fd3000 rw-p 00000000 00:00 0 
7ffff7ff4000-7ffff7ff8000 rw-p 00000000 08:03 3288973                    /home/jords/engcode/workspace/SE370Assignment3/64
7ffff7ff8000-7ffff7ffb000 rw-p 00000000 00:00 0 
7ffff7ffb000-7ffff7ffc000 r-xp 00000000 00:00 0                          [vdso]
7ffff7ffc000-7ffff7ffd000 r--p 00020000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7ffff7ffd000-7ffff7fff000 rw-p 00021000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7ffffffde000-7ffffffff000 rw-p 00000000 00:00 0                          [stack]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]

Program received signal SIGBUS, Bus error.
0x0000000000400abb in main (argc=2, argv=0x7fffffffe058) at A3Program3.c:49
49      printf("%d", mapped_file[0]);
(gdb) 

这让我感到困惑,因为你可以在地图输出中清楚地看到文件已被映射并且地址是正确的 - 但每当我尝试访问它时它都会出错.

编辑:Valgrind:

ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
==16295== 
==16295== Process terminating with default action of signal 7 (SIGBUS)
==16295==  Non-existent physical address at address 0x4024000
==16295==    at 0x400AB7: main (A3Program3.c:49)
==16295== 

更新:所以我已经使用ftruncate调用阻止了它崩溃(感谢:)),但它打印全部为零: __CODE__事件.

Answer 1

问题是你打开O_TRUNC截断它的文件,因为mmap只能映射文件的现有内容,它会给你一个sigbus.truncate()在映射文件之前,使用(和朋友)将文件截断为正确的大小.

从mmap的手册页:

   SIGBUS Attempted access to a portion of the buffer that does not correspond to the file (for example, beyond the end of the file, including the case where another process has truncated the file).